Gas barrier materials and packaging materials making use of them are already well known. For example, gas barrier films are widely used primarily to protect contents as various packaging materials led by food packaging materials and including pharmaceutical packaging materials, and also in the field of industrial materials. As a material that exhibits the most excellent oxygen gas barrier property, an aluminized film can be mentioned. This film, however, involves inter alia a problem in that different from resin films, it is opaque and its content is not visible. Here, representative of resin materials that exhibit gas barrier properties are ethylene-vinyl alcohol copolymer resin (hereinafter abbreviated as “EVOH”) and vinylidene chloride resin (hereinafter abbreviated as “PVDC”).
The above-mentioned EVOH is excellent in hot melt formability, and is therefore used by processing it into single-layer films by melt extrusion or into double-layer films by co-extrusion with a resin such as polypropylene. EVOH is, however, accompanied by a drawback in that it is prone to moisture absorption and is lowered in gas barrier properties under high humidity. Moreover, limitations are imposed on the use of EVOH because it is hardly soluble in organic solvents and cannot be formed into films by a coating method.
On the other hand, PVDC is a resin, which is less temperature-dependent in gas barrier properties compared with EVOH and is formable by a coating method. The coating method has merits in that compared with a co-extrusion method, it requires simpler production equipment and can obtain films at relatively low cost. In addition, PVDC has substantially no hygroscopicity and exhibits good gas barrier properties even under high humidity, and is therefore applicable to various base materials for their coating irrespective of moisture permeability. Various PVDC-coated films produced by methods such as that described above are used as packaging materials for foods centering around those containing abundant water. Further, films with PVDC films laminated therein are used as gas barrier packaging materials for various foods no matter whether they are dry products or wet products.
Packaging materials formed with PVDC, however, involve issues to be described hereinafter. After use, these packaging materials are discarded as domestic waste from households. There is hence an indication that, as PVDC contains chlorine, chlorine gas which occurs upon incineration disposal causes acid rain. It is also indicated that PVDC produces noxious fumes upon burning and its incineration especially at low temperatures is a cause of the production of organic chlorine compounds considered to have strong carcinogenicity. For these issues, there is an outstanding strong desire for a switch to another material.
For these issues, polyvinyl alcohol (PVC)-based films are also well known as another gas barrier material that produces no organic chlorine compound, and films with a water-soluble PVA-based resin coated thereon are available on the market. As PVA is prone to moisture absorption like EVOH, films formed with PVA exhibit highly excellent oxygen gas barrier property under low humidity conditions, but at relative humidities higher than 60%, their oxygen barrier property is drastically deteriorated so that they are considered to have low practical utility under such an environment. The replacement has, therefore, moved ahead only in limited applications.
Concerning improvements in the moisture resistance of EVOH and PVA, numerous studies have been conducted as will be described hereinafter. Contrived include, for example, crosslinking with methacrylic acid (Patent Document 1), the introduction of a second component such as the addition of acrylic acid (Patent Document 2), and a method that provides EVOH or PVA with a crosslinked structure by heat treatment after formation of a film (Patent Document 3), and improvement effects have been obtained to some extent. Such modified EVOH and PVA have, however, not been widely used yet as replacements for PVDC because they require, for example, heat treatment for the production of films.
On the other hand, the developments of gas barrier materials with resins having chemical structures different from olefinic resins are under way. For example, a barrier material with a polyamide resin having amide bonds and a barrier material with a polyurethane resin having urethane bonds have been contrived (Patent Documents 4 and 5). However, these polyamide resin and polyurethane resin need to introduce rigid chemical structures such as meta-xylene skeletons as an essential component, and as a result, the resins have higher melting points. These contrivances are, therefore, accompanied by drawbacks in that they cannot be co-extruded with a general-purpose resin and they are insoluble in organic solvents as in the case of EVOH. It has also been contrived to use the above-mentioned polyurethane resin as an emulsion (Patent Document 6). As a water-based coating formulation, however, a solvent-based coating system cannot be applied as it is. Accordingly, such an emulsion has not substituted yet for a PVDC coating formulation. Gas barrier films also include aluminized films, which have found wide spread utility centrally for snacks. However, as mentioned before, the contents are not visible in this case. Depending on the application, transparent films may be preferred in many instances. The invisibility of the contents may, therefore, become a drawback.
Further, a variety of studies have also been conducted on coating compositions capable of imparting gas barrier properties to various base materials. Concerning, for example, coating compositions making use of an epoxy resin, methods have been proposed to improve the gas barrier properties to oxygen, carbon dioxide and the like by increasing the contents of amine nitrogen in the compositions (Patent Documents 7 and 8). However, further improvements are desired because these coating compositions are not very high in barrier properties and their barrier properties are not high under high humidity conditions. On the other hand, epoxy resin compositions exhibit good performance in gas barrier properties, adhesiveness and chemical resistance, but involve drawbacks that they are short in pot life and poor in workability due to the high reactivity between polyamines and epoxy resins.
Also proposed are a polyurethane resin having gas barrier properties as mentioned above and a gas barrier film containing it (Patent Document 5) and a stretched gas-barrier laminate film making use of a polyurethane-based thermoplastic elastomer (Patent Document 9). However, they are both thermoplastic resins, and are therefore not suited for coating compositions. In addition, these resins are not equipped with any sufficient adhesiveness to base materials. Upon using them as packaging materials for foods and pharmaceuticals, said packaging materials requiring gas barrier properties, it becomes necessary to laminate them as gas barrier layers on conventional general films with an adhesive being additionally applied to opposite sides of the gas barrier layers. Therefore, the above-described materials involve disadvantages in the manufacturing cost of laminated films and the process step of lamination, and moreover, there is a concern about effects on the environment by an increase in waste materials that is considered as a problem these days.
As has been described above, no resins or other materials have been developed yet to completely replace PVDC. It is, accordingly, a current situation that PVDC is still continuously used at present. It is thus practiced in some incineration plants to burn waste materials at temperatures higher than usual in order to decompose noxious fumes that occur upon incineration. However, high-temperature incinerators require more energy than general incineration, and cannot be considered to be preferable in that they increase carbon dioxide emissions which have become a problem in recent years. It is, therefore, required to develop a material that not only suppresses the occurrence of noxious fumes but also takes one step forward and leads to a reduction in carbon dioxide emissions. Under the above-described circumstances, the present inventors came to realize the importance of expanding the range of a study for the development of a new material that leads to a reduction in carbon dioxide emissions, and also of studying the applicability of materials which are new resins different in chemical structure from the conventional urethane resins, for example, like polyhydroxyurethane resins and moreover have not been used as forming materials for gas barrier films to date.